In the nineteen twenties, it was discovered by Becket (U.S. Pat. No. 1,710,445) that the addition of 15 to 40 wt. % molybdenum to nickel resulted in alloys highly resistant to non-oxidizing acids, notably hydrochloric and sulfuric, two of the most important industrial chemicals. Since the least expensive source of molybdenum was ferro-molybdenum, a significant quantity of iron was included in these alloys. At about the same time, it was also discovered by Franks (U.S. Pat. No. 1,836,317) that nickel alloys containing significant quantities of molybdenum, chromium, and iron, could cope with an even wider range of corrosive chemicals. We now know that this is because chromium encourages the formation of protective (passive) films in so-called oxidizing acids (such as nitric), which induce cathodic reactions of high potential. These inventions led to the introduction of the cast HASTELLOY A, B, and C alloys, and subsequently to the wrought B, C, and C-276 alloys. The need to minimize the carbon and silicon contents of such alloys, to improve their thermal stability (Scheil, U.S. Pat. No. 3,203,792) was factored into the composition of HASTELLOY C-276 alloy.
With regard to the quantities of molybdenum and chromium that can be added to nickel, these are dependent upon thermal stability. Nickel itself possesses a face-centered cubic structure, at all temperatures below its melting point. Such a structure provides excellent ductility and resistance to stress corrosion cracking. Thus, it is desirable that alloys of nickel designed to resist corrosion also possess this structure, or phase. However, if the combined additions exceed their limit of solubility in nickel, second phases of a less-desirable nature are possible. Metastable or supersaturated nickel alloys are possible if high temperature annealing (to dissolve unwanted second phases), followed by rapid quenching (to lock in the high temperature structure) are employed. The Ni—Mo alloys and most of the Ni—Cr—Mo alloys fall into this category. The main concern with such alloys is their propensity to form second phase precipitates, particularly at microstructural imperfections such a grain boundaries, when reheated to temperatures in excess of about 500° C., where diffusion becomes appreciable. Such elevated temperature excursions are common during welding. The term thermal stability relates to the propensity for second phase precipitation at elevated temperatures.
In the nineteen fifties, Ni—Mo and Ni—Cr—Mo alloys with low iron contents, covered by G.B. Patent 869,753 (Junker and Scherzer) were introduced, with narrower compositional ranges and stricter controls on carbon and silicon, to ensure corrosion resistance yet minimize thermal instability. The molybdenum range of the nickel-molybdenum (Ni—Mo) alloys was 19 to 32 wt. %, and the molybdenum and chromium ranges of the nickel-chromium-molybdenum (Ni—Cr—Mo) alloys were 10 to 19 wt. % and 10 to 18 wt. %, respectively. These led to the introduction of wrought HASTELLOY B-2 and C-4 alloys in the nineteen seventies.
Since then, it has been discovered that HASTELLOY B-2 alloy is prone to rapid, deleterious phase transformations during welding. To remedy this, HASTELLOY B-3 alloy, the phase transformations of which are much slower, was introduced in the nineteen nineties after discoveries by Klarstrom (U.S. Pat. No. 6,503,345). With regard to recent developments in the field of Ni—Cr—Mo alloys, these include HASTELLOY C-22 alloy (Asphahani, U.S. Pat. No. 4,533,414), HASTELLOY C-2000 alloy (Crook, U.S. Pat. No. 6,280,540), NICROFER 5923 hMo (Heubner, Köhler, Rockel, and Wallis, U.S. Pat. No. 4,906,437), and INCONEL 686 alloy (Crum, Poole, and Hibner, U.S. Pat. No. 5,019,184). These newer alloys require molybdenum within the approximate range 13 to 18 wt. %, and chromium within the approximate range 19 to 24.5 wt. %.
With a view to enhancing the corrosion performance of the Ni—Cr—Mo alloys, additions of tantalum (of the so-called reactive element series) have been used. Notably, U.S. Pat. No. 5,529,642 describes an alloy containing from 1.1 to 8 wt. % tantalum. This has been commercialized as MAT-21 alloy.
Although the Ni—Mo alloys possess outstanding resistance to non-oxidizing acids (i.e. those which induce the evolution of hydrogen at cathodic sites), they are intolerant of additions, residuals, or impurities which result in cathodic reactions of higher potential. One of these so-called “oxidizing species” is oxygen, which is hard to avoid. While the Ni—Cr—Mo alloys can tolerate such species, they do not possess sufficient resistance to the non-oxidizing acids for many applications. Thus there is a need for materials which possess the attributes of both the Ni—Mo and Ni—Cr—Mo alloys.
Materials with compositions between those of the Ni—Mo and Ni—Cr—Mo alloys do exist. For example, a Ni—Mo—Cr alloy containing approximately 25 wt. % molybdenum and 8 wt. % chromium (242 alloy, U.S. Pat. No. 4,818,486) was developed for use at high temperatures in gas turbines, but has been used to resist aqueous environments involving hydrofluoric acid. Also, B-10 alloy, a nickel-based material containing about 24 wt. % molybdenum, 8 wt. % chromium, and 6 wt. % iron was promoted as being tolerant of oxidizing species in strong non-oxidizing acids. As will be shown, however, the properties of these two Ni—Mo—Cr alloys are generally similar to those of the Ni—Mo alloys, and do not provide the desired versatility.